1. Field of the Invention
The present invention relates to a line light source, which is employed to irradiate readout light onto an image recording medium, to read out image information recorded thereon. Particularly, the present invention relates to a line light source that employs an EL layer, and to an image information readout system that employs such a light source.
The present invention also relates to a scanning exposure apparatus, which is employed to perform scanning exposure on an image recording medium, to read out image information recorded thereon. Particularly, the present invention relates to a scanning exposure apparatus that sequentially emits linear light beams from a panel light source, and performs scanning exposure in a scanning direction perpendicular to the longitudinal direction of the linear light beams. Further, the present invention relates to an image information readout system that employs such a scanning exposure apparatus.
2. Description of the Related Art
In the field of medical X-ray imaging, there are known image information readout systems, such as that disclosed in U.S. Pat. No. 6,268,614. This image information readout system employs an X-ray sensitive photoconductive material, such as a selenium plate formed of a-Se, as an electrostatic recording medium. Radiation, such as X-rays, bearing image information is irradiated onto the electrostatic recording medium, and latent image charges that bear the radiation image information are accumulated therein. An electrostatic latent image borne by the latent image charges is read out by scanning the electrostatic recording medium with linear light beams emitted from a line light source. The scanning causes electric current to be generated within the electrostatic recording medium. The electric current is detected via stripe electrodes. This configuration is adopted to reduce a radiation dose received by a subject, and also to improve diagnostic performance.
There are known other image information readout systems, as disclosed, for example, in U.S. Patent Application Publication No. 20030057386. This image information readout system employs stimulable phosphors as a recording medium. Stimulable phosphors store a portion of radiation energy irradiated thereon, and generate stimulated phosphorescence when scanned by linear light beams emitted from a line light source. Image information borne by the stimulable phosphors is read out by detecting the stimulated phosphorescence. Photodiodes, CCD's, or detectors having photoconductive layers that exhibit conductivity when irradiated with the stimulated phosphorescence may be employed as the detecting portion for detecting the stimulated phosphorescence. The detectors may either be in the form of a panel, or in linear form.
In the image information readout systems described above, scanning exposure apparatuses, in which line light sources are mechanically moved to perform scanning exposure with linear light beams, are employed. However, in the case that image information is read out employing such scanning exposure apparatuses, it is difficult to move the line light sources at high speed, therefore preventing acceleration of readout speed. For this reason, a scanning exposure apparatus has been proposed in U.S. Pat. No. 6,376,857.
This scanning exposure apparatus comprises a panel light source constituted by a great number of line light sources, which are arranged parallel at substantially equidistant intervals. Linear light beams are sequentially emitted by the panel light source at different timings, thereby performing scanning exposure. The panel light source comprises: light transmissive linear electrodes; a rear surface electrode constituted by a metal plate; and an EL layer provided between the linear electrodes and the rear surface electrode. Linear light beams are sequentially emitted, by causing electric current to flow through the EL layer between the linear electrodes and the rear surface electrode. ITO (Indium Tin Oxide) is utilized as the material of the light transmissive linear electrodes.
In scanning exposure apparatuses that employ light transmissive linear electrodes, an EL layer, and a planar metallic electrode as that described above, generally, the resolution during readout of image information from an image recording medium is inversely proportional to the gaps between the linear electrodes. Therefore, it is desirable that the line widths of the linear electrodes are narrow. Currently, development of scanning exposure apparatuses, in which linear electrodes have line widths on the order of several tens of μm is being anticipated. Meanwhile, it is desirable that the thickness of the light transmissive linear electrodes is thin, inorder to improve the light transmissivity therethrough. A thickness of 1 μm or less is preferable, and a thickness of 0.5 μm or less is further preferable. However, it is often the case that materials, which are capable of being formed into light transmissive linear electrodes, possess comparatively high specific resistances. For example, the specific resistance of ITO, which constitutes the light transmissive linear electrodes of the aforementioned scanning exposure apparatus, is approximately 4×10−4 Ω. For this reason, in the case that the size of an image recording medium is 430 mm×430 mm, and the linear electrodes have line widths of 50 μm, thicknesses of 1 μm, and lengths of 430 mm, the resistance in the longitudinal direction thereof may be obtained as follows. First, the sheet resistance of ITO having a thickness of 1 μm is:4×10−4 Ω·cm/1×10−4 cm=4 Ω/sqAccordingly, the resistance of the linear electrodes in the longitudinal direction thereof may be calculated by substituting the above value in an equation (sheet resistance×length)/line width, to yield:(4 Ω/sq×430 mm)/(50×10−3 mm)≈34 KΩIf an electric current of 2 mA is caused to flow through the linear electrodes, a potential difference of 68V is generated at the two ends thereof. There is a problem that the amount of light generated in the EL layer at the ends of the linear electrodes, to which drive current is not connected, is significantly reduced, due to a voltage drop. In the case that the thickness of ITO is made as thin as 0.4 μm in order to improve the light transmissive property thereof, the resistance of the linear electrodes formed thereby will be approximately 86 KΩ. FIG. 20 is a graph that illustrates the relationship between the distance x from an end of a linear electrode, to which voltage is applied, and a voltage V(x), which is applied to an EL layer. The graph assumes that an planar aluminum electrode (specific resistance: 2.7×10−6 Ω·cm) having a thickness of 0.1 μm is employed as a rear surface electrode, and that a voltage of 65V is applied to one end of the linear electrode. As can be seen from FIG. 20, the voltage drops to approximately 20% at a portion about 100 mm distant from the end of the linear electrode. FIG. 21 is a graph that illustrates the relationship between the distance x from an end of a linear electrode and the amount of electric current D(x), which flows through an EL layer. It can be seen from this graph that hardly any electric current flows through the EL layer at a portion 100 mm distant from the end of the linear electrode. For this reason, the EL layer does not emit light at a portion 100 mm distant from the end of the linear electrode. That is, in a scanning exposure apparatus configured as described above, it is difficult to cause an EL layer to emit light at the end of an electrode, to which drive current is not connected.